Gizmodo Australia claims to have a training document which details the rumoured Google Nexus tablet. It's built by Asus, 7", 1280x800, 1.3Ghz Tegra 3 processor, 1GB of RAM, GeForce '12-core' GPU (whatever the heck that means). The 8GB variant will be $199, the 16GB one $299. It looks quite decent, and will run Android 4.1 Jelly Bean. Interestingly, the document also states that going forward, Google will take care of Android updates directly - however, it's not made clear if that's for this device only, or for all Android devices. The "going forward" bit would suggest all devices, but as much as I hope so, it seems unlikely.

It is technically impossible. Microsoft force some standard technologies for use with WindowsRT, but Google does not for Android. So, given the fact that Linux modules/drivers are not backward/forward compatible, there is no way they could support all devices at once.